When an alternative energy source such as a photovoltaic array is installed on a homeowner's roof, it is often important to provide a path for the generated electricity to flow back to the utility whenever the power generated by the photovoltaic array is greater than the homeowner's demands. This is especially important in net metering jurisdictions as it allows the homeowner to make money by selling their generated power back to the utility.
Unfortunately, feeding power from a photovoltaic array back through the circuit breakers in the main breaker box can cause problems. Specifically, the photovoltaic array's power could overload the breaker box's bus bars during periods of high current draw. Bus bars have maximum amperage ratings that, when exceeded, can damage the bus bars. Therefore, the combined power fed into the bus bars from both the utility and the photovoltaic array could exceed the bus bar rating.
There are also various national, state, and municipal electrical guidelines and regulations that impose restrictions on the amount of power that can be backfed to utilities. For example, section 690.64(B)(2) of the 2008 National Electric Code, adopted by many states, requires that the sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120% of the rating of the busbar or connector.” As a result of these restrictions, homeowners with alternative energy systems are often unable to use those systems to satisfy their load requirements and may also be required to limit the amount of power their systems backfeed to the utility.
One solution to this problem has been to upgrade (i.e., replace) the entire breaker box with a new box having bus bars that have higher amperage ratings. However, this approach adds cost and time to the photovoltaic system installation, and on permit approvals. Solar installers (and homeowners) often do not want to install a new breaker box because of the disruption. Another approach has been to install circuit breakers for the photovoltaic array in a separate secondary breaker box. The problem with this approach is both the cost in installing the secondary breaker box (dedicated only to the photovoltaic array) and the additional space required to install this secondary sub-panel breaker box.
Another existing solution to the problem of overloading the bus bars has been to install the utility input breaker and the solar input breaker at opposite ends of the breaker box's bus bars (i.e., the building's actual load is wired between the two breakers). This prevents the current from the utility and the photovoltaic system from adding up and overloading the bus bars. Instead, the current delivered to the building's actual loads is separated into current from the photovoltaic array travelling through one portion of the bus bars and current from the utility mains travelling through another portion of the bus bars. Unfortunately, the problem with this approach is that any inadvertent re-positioning of the two power breakers away from the ends of the bus bars could result in a situation where the building load is not positioned between the utility and photovoltaic inputs, which could overload the bus bars. Simply put, this approach relies on installers correctly wiring the breaker box initially, and not re-positioning the breakers after their installation. Accidental re-positioning of the breakers by an electrician at a later time could be problematic. In addition, although this solution is acceptable under most codes, the photovoltaic system breaker's maximum amperage can still be limited by the 120% rule in section 690.84(B)(2) of the 2008 National Electric Code.
Another solution to overloading bus bars involves a specially designed breaker. It can act as a supply-side line tap in which power from utility main and photovoltaic systems make separate attachments to the breaker. Power can then flow from the photovoltaic system through internal connections of the breaker to the utility mains with no involvement of the breaker box bus bars. An example of such a system is a breaker system sold by Q-Factory 33 under the trademark B3 BYPASS. The B3 Bypass breaker system has a first breaker between the utility mains and the bus bars, and a second breaker between the photovoltaic system and the utility mains. One limitation of the B3 Breaker system is that it requires two internal circuit breakers within a single housing.